1. Field of the Invention
Embodiments of the present invention relate to laser scanner projection systems. More specifically, embodiments of the present invention relate to laser projection systems and related methods that reduce the appearance of speckle that may be visible in a laser projection image by the use of a periodic phase mask.
2. Technical Background
Speckle may result whenever a coherent light source is used to illuminate a rough surface, for example, a screen, or any other object that produces a diffused reflection or transmission. Particularly, a multitude of small areas of a screen or other reflecting object scatter light into a multitude of reflected beams with different points of origination and different propagation directions. Speckle causes high spatial frequency noise in the projected image. At an observation point such as an observer's eye or a camera's sensor, these beams can interfere constructively to form a bright spot, or destructively to form a dark spot, producing a random granular intensity pattern known as speckle.
Speckle may be characterized by its grain size and contrast, where contrast is usually defined as a ratio of standard deviation to mean light intensity in the observation plane. For a large enough illuminated area and a small enough individual scattering point size, speckle will be “fully developed,” with a brightness standard deviation of 100%. If an image is formed on a screen using laser beams, such granular structure will represent noise or a severe degradation of the image quality. Although this noise may not be a significant problem when a projector is used to display images, it can be a serious issue when the projector is used to display high spatial frequency content, such as text.
The general concept of using diffusers to minimize speckle consists of projecting an intermediate laser image over a small sized diffusing surface, and using projection optics to re-image that intermediate laser image on the final screen. By rapidly moving the diffuser, the phase of the electric field is scrambled over time, which changes the perceived speckle pattern. If the diffuser is moving or vibrating fast enough, the perceived speckle pattern will also change at high frequencies and such pattern changes will be time averaged by the eye. To work efficiently, multiple speckle frames need to be created over the integration time of the eye, which is typically in the order of 50 Hz.
Although rapidly moving the diffuser may provide speckle reduction, it requires an expensive and complicated mechanism to move the phase mask laterally at a relatively high speed.